I used to use the D90 for event coverage and moved to a D700 (similar to the D300 in terms of controls) -- there's a big difference in way you control important settings that might take some getting used to, though you might love the direct controls of the D300 after a while.
AF mode, AF point selection, metering pattern -- some of the settings that D300 has direct switches on the body for. With the D90 (and the D7000 & D5000) you'll need to press a button & turn a dial (or go into a menu) for these. Your preference really, and dependent on how you shoot. I'd think you'd be more comfortable with the D90 or D7000 for a start.
The D90 / D7000 do give you a bit more direct control than the D5000 (2 dials for a start), and are lighter than the D300. Being a newer model, the D7000 does perform very well (speed, high ISO, 30+ AF pts etc). And one thing I really like is the 2 card slots (AFAIK only the pro bodies, D7000 & D300s have this, the D300 does not) -- anyone who has experienced card trouble can appreciate having an immediate backup!
You may want to rent to try them out in actual use.